System and method for driving multiple pumps electrically with a single prime mover

ABSTRACT

A converterless motor-driven pump system includes an off-grid prime mover. The prime mover includes a rotational driveshaft that operates in response to a throttle or fuel input controller to control a rotation speed of the prime mover driveshaft. Operation of the throttle or fuel input controller is based on desired output characteristics of a pumping load. One or more electric power generators are driven by the off-grid prime mover to generate AC or DC power on an electrical bus shared by a plurality of variable speed electric motors. A plurality of pumps is connected to a common manifold shared by the plurality of pumps. The plurality of pumps is driven by the plurality of variable speed electric motors to generate a desired wellhead pressure or pumping load flow rate via the shared common manifold.

BACKGROUND

The subject matter of this disclosure relates generally to motor-drivenpumps, and more particularly, to a multiplicity of motor-driven pumpsbeing fed from a singularity of generators tied to a single, throttlecontrolled prime mover.

A common way to drive multiple pumps uses multiple prime movers. The gasfracking industry, for example, generally employs a single diesel enginepowering a single pump through a mechanical gear box. This techniqueemploys many engines, gearboxes and pumps on many trailers, resulting ina very busy and very crowded pumping structure.

FIG. 1 illustrates a conventional system 1 that is known in the gasfracking industry for operating a pump. The system 1 uses a single primemover including a diesel engine 2 that is fueled by either natural gasor diesel fuel to drive a fracking pump 3. Each fracking pump 3 ismechanically coupled via a transmission and corresponding gearbox 5 to asingle reciprocating pump 3 to achieve optimization of the pumpingsystem 1. System 1 is generally disposed on a single truck trailer. Aplurality of such pump systems 1 together operate to provide a combinedpressure in a common high pressure manifold 6 that supplies a desiredpressure to a wellhead. A typical application may include about 16 suchtruck trailers, each including system 1, and that are backed up to onewell head for fracking.

Another approach to driving pumps is through the use of electric motordriven pumps such as electric submersible pumps (ESPs). A conventionalsystem in the oil and gas industry employs a variable speed drive (VSD)that is fed by a fixed frequency AC supply to drive a single ESP. TheVSD synthesizes voltages and currents of such frequency as is necessaryto operate the pump in the desired manner. In the oil and gas industry,the voltage output by the VSD is usually stepped up to a medium voltageusing a transformer because high voltage motors are deployed in wells toreduce the size of the power cable needed to supply the motor. Similarto the gas fracking industry that generally requires each pump have adedicated diesel engine, each ESP generally requires a dedicated VSD.

FIG. 2 illustrates a conventional system 10 that is known in the oil andgas industry for operating electric submersible pumps (ESPs) 12 in anoff-grid application. One or more prime movers that are directly coupledto generators 14 produce an AC voltage having a fixed frequency andamplitude to supply one or more electrical loads 15. The prime mover(s)may comprise, for example, a reciprocating engine that is fueled byeither natural gas or diesel fuel, or a turbine. The generated AC poweris fed to a VSD 16 that is responsible for regulating the operation ofthe ESP 12 subsequent to stepping up the AC voltage to a medium voltagelevel that is supplied to ESP motor 18 via a suitable transformer 19.

In view of the foregoing, there is a need in the gas fracking industryto provide a pumping system that is less complex, less costly, and thathas a smaller footprint. The pumping system should eliminate thenecessity for using a plurality of diesel engines to power a pluralityof pumps, and should also operate without the need to employ VSDs orother electronic controls to monitor and vary the voltage/frequency tothe load.

BRIEF DESCRIPTION

According to one embodiment, a motor-driven pump system comprises:

-   -   a single off-grid prime mover, the single off-grid prime mover        comprising a rotational driveshaft and operating in response to        a throttle or fuel input controller to control a rotation speed        of the prime mover driveshaft;    -   one or more electric power generators driven by the single        off-grid prime mover rotational driveshaft to generate AC or DC        power on a shared electrical bus;    -   a plurality of variable speed motors configured to receive        electric power via the shared electrical bus; and    -   a plurality of pumps connected to a shared high pressure        manifold and driven by the plurality of variable speed motors to        generate a desired wellhead pressure via the shared high        pressure manifold.

According to another embodiment, a method of operating a motor-drivenpump system comprises:

-   -   controlling a driveshaft rotation speed of an off-grid prime        mover in response to a throttle or fuel input controller,        wherein operation of the throttle or fuel input controller is        based on known characteristics of a pumping load;    -   controlling AC or DC power supplied to a shared electrical bus        from at least one electric power generator in response to a        driveshaft rotation speed of the off-grid prime mover;    -   controlling a speed of a plurality of variable speed motors        directly in response to the AC or DC power on the electrical bus        shared by the plurality of variable speed motors; and    -   generating a desired wellhead pressure or pumping load flow rate        via a plurality of pumps driven by the plurality of variable        speed motors and connected to a high pressure manifold shared by        the plurality of pumps.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings,wherein:

FIG. 1 illustrates a conventional engine-gearbox-pump system that isknown in the gas fracking industry;

FIG. 2 illustrates a conventional electrical submersible pump (ESP)system that is known in the oil and gas industry;

FIG. 3 illustrates multiple motor-driven pumps powered from a sharedelectrical bus that is supplied by a single prime mover/generatoraccording to one embodiment;

FIG. 4 illustrates multiple groups of motor-driven pumps, each grouppowered from a corresponding shared electrical bus that is supplied by asingle prime mover/generator according to another embodiment; and

FIG. 5 is a graph illustrating control characteristics for oneembodiment of a gas turbine engine/prime mover for both a conventionalsingle speed (60 Hz) mode and a variable speed mode using directelectrical coupling to a plurality of motors.

While the above-identified drawing figures set forth particularembodiments, other embodiments of the present invention are alsocontemplated, as noted in the discussion. In all cases, this disclosurepresents illustrated embodiments of the present invention by way ofrepresentation and not limitation. Numerous other modifications andembodiments can be devised by those skilled in the art which fall withinthe scope and spirit of the principles of this invention.

DETAILED DESCRIPTION

The embodiments described herein are directed to control of motor-drivenpumps in applications that are operating independently of a utilitypower grid, and combine the control of a single prime mover and one ormore AC or DC generators to provide electric power to a plurality ofmotor-driven pumps in a manner that reduces system complexity, cost andfootprint size. Such embodiments are particularly useful in the gasfracking industry where the usual control objective is to regulatewellhead pressures and/or flow rates.

FIG. 3 is a motor-driven pumping system 20 illustrating a plurality ofmotor-driven pumps 22, according to one embodiment. Each pump 22 isdriven by a corresponding electric motor 24. The electric motors 24 arepowered from a shared electrical bus 26 that is supplied by a singleprime mover 28 mechanically coupled to one or more electric generators30. Although shown as an AC powered system, the principles describedherein are just as easily applied to a DC powered system in which motorwinding field/voltage regulator controls 32, 38 are not required.

The prime mover 28 may comprise a fuel based engine or othercontrollable source of rotational energy. In some applications, theprime mover 28 may comprise, without limitation, a gas turbinegenerator, a diesel engine, or a reciprocating engine that is fueled byeither natural gas or diesel fuel. Although only a single prime mover 28is depicted in FIG. 2, other applications may comprise multiple primemovers 28 so long as each prime mover 28 and associated generator(s) 30supplies power to a corresponding plurality of motor-driven pumps on acorresponding shared electrical bus, such as the embodiment describedherein with reference to FIG. 3.

According to one embodiment, the electric generator 30 is an ACgenerator comprising a field regulator 32. According to another aspect,the generator 22 may be a permanent magnet generator that does notrequire excitation, and therefore does not require a field/voltageregulator, as stated herein. It can be appreciated that use of apermanent magnet generator would further simplify the motor-drivenpumping system 20 without sacrificing performance. The electricgenerator 30 then generates AC or DC power, depending on the particularapplication. The magnitude of the voltage output by the electricgenerator 30 is generally, but not necessarily, proportional to therotation speed of the prime mover driveshaft 34.

Further, the electric generator 30 is electrically coupled to theplurality of respective AC or DC electric motors 24 via the sharedelectrical bus 26 that is common to the plurality of electric motors 24.The motor-driven pumping system 20 may comprise more than one electricgenerator 22, depending on the particular application. Regardless, anyplurality of electric generators 22 that may be employed functiontogether as a singularity of generators to supply electric power to theshared electric bus 26. In this way, each motor operates at the samefrequency and/or voltage to supply power to the plurality of pumps 22such that the plurality of pumps 22 together control a single wellheadpressure or pumping load flow rate.

According to one aspect, the need for variable speed and output pumpingpower is accomplished by control of the prime mover's throttle or fuelinput controller/mechanism 36. The electric generator's voltage controlelement/regulator 32 is locally implemented and utilizes the rotationspeed of the prime mover's driveshaft as a control input via a generatorcontrol element 38. Although described in terms of a single prime mover28 and a single electric generator 30, the principles described hereinapply as well to a plurality of prime movers, generators, load motorsand rotational loads/pumps, such described herein with reference to FIG.3. It can be appreciated that such elements may be mechanicallyconnected in various fashions, but the common electricalconnection/shared bus 26 is assumed; and pumping power/motors 24 arecontrolled through the prime mover(s) throttle control/mechanism 36through the use of feedback and/or feedforward information such as,without limitation, wellhead pressure, and/or pumping load flow.

According to one aspect, the throttle control/mechanism 36 may belocally operated manually by an operator that has knowledge of pumpingload characteristics, allowing the operator to manually control theelectrical frequency or pumping speed in which the power to the load isapproximated by a quadratic function of the electrical frequency orpumping speed. FIG. 5 is a graph 90 illustrating control characteristicsfor one embodiment of a gas turbine engine/prime mover for both aconventional single speed (60 Hz) mode and a variable speed mode usingdirect electrical coupling to a plurality of motors. The prime moveroutput power 92 and fuel efficiency 94 change in response to the speedof the prime mover. Both the output power 92 and fuel efficiency 94 areaffected in different ways as depicted for variable speed operation 96and constant speed operation 98. It can be appreciated that DC voltageeffects DC motor speed and pump speed when using DC motors.

According to another aspect, the throttle control/mechanism 36 may beremotely operated from a programmable controller/computer operatingcenter 31 where an operator may transmit a command to the throttlecontrol/mechanism 36, allowing the operator to control the wellheadpressure and/or pumping load flow rate remotely. According to anotheraspect, the prime mover throttle control/mechanism 36 relies on feedbackinformation from the electrical generator(s) 30, the variable speedmotor(s) 24, and/or the pump(s) 22 in order to control the wellheadpressure and/or pumping load flow rate(s).

The motor-driven pumping system 20 thus uses and controls a plurality ofpumps 22 to achieve a desired pressurization and hydraulic fracturing ofa gas well. According to one aspect, the motor-driven pumping system 20is mobile via a plurality of trailers 42, and advantageously provides asmall footprint near the wellhead 40 by installing two or more pumps 22and motors 24 per trailer 42, resulting in fewer prime movers of largerpower rating either near the wellhead 40, or remote from the well headsite. A corresponding manual disconnect mechanism 50 is employed tocouple each electric motor 24 to the shared electrical bus 26. It can beappreciated that small prime movers are more efficient and can belogistically easier to employ and maintain, particularly when using asimple electrical connection to the loads.

Depending on the selection of the prime mover(s) 28 and the generator(s)30, it may be desirable to use a gearbox to match the shaft speeds ofthe prime mover(s) 28 and generator(s) 30. It is preferable to use afixed ratio gearbox to keep the system 20 as simple as possible.According to one aspect, the motor-driven pump(s) 22 are located on thesame trailer 42 transporting the pump motors 24.

According to one embodiment, each motor 24 is mechanically coupled via atransmission 44 and corresponding gearbox 46 to a single reciprocatingpump 22 to achieve optimization of the motor-driven pumping system 20.The plurality of reciprocating pumps 22 together operate to provide acombined pressure in a common high pressure manifold 48 that supplies adesired pressure to the wellhead 40. The wellhead pressure may bemonitored via a pressure sensor 52 at or near the wellhead. Themotor-driven pumping system 20 may further employ relay and protectionequipment 50 that will shut down the system 20 due to predeterminedoverload/fault conditions that may occur during operation of the system20.

The motor-driven pumping system 20 advantageously i) eliminates the needfor a variable speed drive or a plurality of variable speed drives andtransformer, simplifying the system, resulting in improved systemreliability, ii) can optionally use pumped gas via the system pump(s) 22as the fuel to run the prime mover(s) 28, resulting in very low fuelcosts, iii) operates independently of a utility power grid; iv)eliminates the need for a multi-speed gear box between the engine primemover(s) 28 and the pump(s) 22; v) eliminates a multitude ofengine-gearbox-pump systems in favor of a single large prime mover 28having a single control point; and vi) allows for a large prime mover 28to be located some distance away from the pump(s) 22 and wellhead 40,thus giving more access to productive equipment e.g. slurrydistribution, pumps, valves, safety equipment, etc.

It can be appreciated that there may be reasons to retain a transformerbetween the generator 28 and the motor driven pump(s) 22. Such reasonsmay include, without limitation, minimizing system cost and/ormaximizing operational flexibility. According to one aspect, thedecision to retain or remove the transformer(s) from the system 20 maybe made on the basis of system optimization rather than conceptualoperation of the system 20.

It can further be appreciated that the pump motor(s) 22 may be anyelectric motor that can be line started, including not only inductionmotors, but also a special class of permanent magnet motors known asline-start permanent magnet motors.

FIG. 3 is a simplified system diagram illustrating a motor-drivenpumping system 60 using multiple groups 62, 64 of motor-driven pumps 22,each group 62, 64 of pumps 22 powered from a corresponding sharedelectrical bus 66, 68 that are each supplied by a corresponding singleprime mover/generator combination 70, 72 and 74, 76, according toanother embodiment. A programmable controller 80 is programmed tomonitor desired operating conditions such as, without limitation, wellhead pressure(s) and/or pumping load flow rates. The programmablecontroller 80 communicates control signals to the corresponding throttleor fuel input controls/mechanisms 36, 37 that control the fuel flow tothe respective prime movers 70, 74.

Each prime mover/generator combination 70, 72 and 74, 76 may becontrolled independently of each other based on control signals receivedfrom the programmable controller 80 that may be implemented as a singlecontrol unit or may be implemented in numerous ways as a distributedcontroller, based on the particular application. In this way, one group62 of motor-driven pumps 22 can function to generate a desired wellheadpressure for a first gas well; while a second group 64 of motor-drivenpumps 22 can function to generate a desired wellhead pressure for asecond gas well independent of the first group 62 of motor-driven pumps22. The number of motor-driven pump groups is limited only by theparticular application, and is theoretically unlimited. According to oneembodiment, the programmable controller 80 receives a single feedbacksignal from each group 62, 64 of motor-driven pumps 22 rather than aplurality of signals comprising a signal associated with each individualmotor-driven pump 22. In this way, the programmable controller 80controls each group 62, 64 of motor-driven pumps 22 based on a singlefeedback signal to achieve a desired pumping load characteristic suchas, without limitation, wellhead pressure or pumping load flow rateassociated with the corresponding high pressure manifold 47, 49. Thus,control of each group 62, 64 of motor-driven pumps 22 is based on acorresponding feedback signal that is common to all of the motors 24 andpumps 22 within the corresponding group.

The programmed controller 80 may include a processor and a memorydevice. The processor includes any suitable programmable circuit whichmay include one or more systems and microcontrollers, microprocessors,reduced instruction set circuits (RISC), digital signal processors(DSPs), application specific integrated circuits (ASIC), programmablelogic circuits (PLC), field programmable gate arrays (FPGA), and anyother circuit capable of executing the functions described herein. Theabove examples are exemplary only, and thus are not intended to limit inany way the definition and/or meaning of the term “processor.” Thememory device includes a computer readable medium, such as, withoutlimitation, random access memory (RAM), flash memory, a hard disk drive,a solid state drive, a diskette, a flash drive, a compact disc, adigital video disc, and/or any suitable device that enables theprocessor to store, retrieve, and/or execute instructions and/or data.

In the exemplary embodiment depicted in FIG. 3, programmed controller 80includes a plurality of control interfaces 82, 84 that are coupled toprime mover throttle or fuel input controls/mechanisms 36, 37 to controla fuel flow rate for each respective prime mover 70, 74. In addition,programmed controller 80 also includes a sensor interface 86 that iscoupled to at least one sensor 52 such as shown and described withreference to FIG. 2. Each sensor 52 may transmit a signal continuously,periodically, or only once and/or any other signal timing that enablesprogrammed controller 80 to function as described herein. Moreover, eachsensor 52 may transmit a signal either in an analog form or in a digitalform.

Programmed controller 80 may also include a display and a userinterface. The display, according to one embodiment, includes a vacuumfluorescent display (VFD) and/or one or more light-emitting diodes(LED). Additionally or alternatively, the display may include, withoutlimitation, a liquid crystal display (LCD), a cathode ray tube (CRT), aplasma display, and/or any suitable visual output device capable ofdisplaying graphical data and/or text to a user.

Various connections are available between the programmed controller 80and each throttle or fuel input control/mechanism 36, 37. Suchconnections may include, without limitation, an electrical conductor, alow-level serial data connection, such as Recommended Standard (RS) 232or RS-485, a high-level serial data connection, such as Universal SerialBus (USB) or Institute of Electrical and Electronics Engineers (IEEE)1394 (a/k/a FIRE WIRE), a parallel data connection, such as IEEE 1284 orIEEE 488, a short-range wireless communication channel such asBLUETOOTH, and/or a private network connection, whether wired orwireless.

In summary explanation, a motor-driven pumping system 20, 60 eliminatesthe need for a variable speed drive and, potentially, its associatedtransformer from a motor-driven pump system, resulting in a simplersystem that reduces capital expense, weight and system footprint. Themotor-driven pumping system 20, 60 employs a remotely located primemover 28, improving access to the well head 40 region for frackingequipment and safety. The use of power generated on-site advantageouslyreduces the time it takes to put a well into production resulting fromdelays in getting the utility to install requisite power lines. Further,the use of natural gas produced by the well itself advantageouslyreduces the operating expense. The motor-driven pumping system 20, 60 isparticularly advantageous in that a single prime mover controls aplurality of motor-driven pumps via a shared electrical bus and a sharedhigh pressure manifold to manage a wellhead pressure and/or pumping loadflow rate associated with a gas well that is common to all themotor-driven pumps.

Since the output of the generator(s) 30, 72, 76 is substantiallysinusoidal when compared with the output of a variable speed drive, afilter is not required between the generator(s) and the motor(s) 24. Theoutput of a variable VSD, for example, contains significant highfrequency content, the result of chopping up DC voltage/current toproduce AC voltage/current. This chopping action disadvantageouslycreates high frequency components called harmonics that are detrimentalto the motor driving the pump. A filter is usually installed between theVSD and the motor; however, anecdotal data suggest that even such afilter may not always adequately filter out the harmonics, leading toaccelerated aging of the insulation systems in the transformer, cable,and motor. This disadvantageously reduces the life of the system

A VSD also draws nonsinusoidal currents from its supply, unless anactive front end is applied to the VSD. These resulting harmonics aredetrimental to the generator supplying the VSD. Many system designsoversize the generator so that it can better tolerate the harmoniccurrents drawn by the VSD. Other system designs will use an active powerfilter to source the harmonic currents drawn by the VSD, therebyalleviating the generator from having to supply them. Either of suchapproaches adds to the cost and complexity of the system. Further, VFD'soccupy space near the wellhead 40, and can be a reliability problem.

The principles described herein with reference to the variousembodiments include reduced capital expense and more timely wellproduction. The off-grid motor-driven system embodiments describedherein advantageously allow putting a well into production sooner sincethere is frequently a substantial waiting period for the utility toinstall supply lines to the well site, as stated herein. At such time asutility power is available, the well operator can remove the prime moverand generator, replacing them with a variable speed drive andtransformers if desired.

However, this is typically not done in the gas fracking industry sincefracking is done by bringing in tractor trailer trucks with adiesel-transmission-pump system on each bed 42. Such tractor trailertrucks consume substantial amounts of fuel and emit substantial amountsof smoke and noise. It is easily appreciated that a wellhead 40 may getvery crowded with, for example, 10-12 fracking trucks in close proximityto the wellhead 40.

Looking again at FIG. 2, the prime mover driveshaft 34 is coupleddirectly or indirectly to the generator 30; while the generator 30 iselectrically coupled to a plurality of motors 24 that may be line startmotors such as induction motors, permanent magnet motors, or DC motorsused in association with a DC generator system, via a shared electricalbus 26; and each motor driveshaft is directly or indirectly coupled to acorresponding well pump 22, as depicted in FIG. 2. The prime mover 28 isturned-on to rotate its driveshaft 34, causing the generator 30 toproduce AC or DC power sufficient to power the plurality of motors 24,that subsequently drive the plurality of well pumps 22. It can beappreciated that if the generator derives variable DC electricity, themotor(s) must be a DC motor(s); and that if the generator producesvariable frequency AC, the motor(s) can be either an induction,permanent magnet or wound field synchronous motor(s).

The present inventors realized that the engine throttle control element36 is the only control element necessary to control the plurality ofwell pumps 22, recognizing the nature of the pumping load, where powerto the load is typically a quadratic function of speed/frequency.Therefore, the engine throttle control element 36 may be controlled inan open loop mode, a closed loop mode, or a combination thereof, eithermanually or automatically, in which the engine throttle setting is basedon a desire to do more or less pumping at the load. The desire to domore or less work is based on the nature of the pumping load, as statedherein, and can be compared with driving a motor vehicle where in orderto get home faster, one need only press down harder on the accelerator(throttle). It is not necessary to measure the output power andprecisely set the accelerator (throttle). The speed of the motor vehicleis regulated by approximately pressing on the accelerator.

The engine throttle control element 36, when operated in a closed loopmode, may operate in response to one or more desired operationalcharacteristics, including without limitation, electric motor voltage,electric motor frequency, pump speed/rotational frequency, wellheadpressure(s), pumping load flow rates, and pump operating point(s), amongothers.

Since some applications may employ a permanent magnet generator thatdoes not require excitation, it can be appreciated that a generatorexciter will not be required in such applications. The use of apermanent magnet generator further simplifies the motor-driven pumpingsystem 20 without sacrificing performance, as stated herein.

Although particular embodiments have been described herein withapplication to electric motor-driven gas well fracking pumps, theprinciples described herein can just as easily be applied to otherapplications including without limitation, geothermal applications. Insuch applications, gas turbines, reciprocating engines, or otherrotational energy sources can be employed to rotate the generator.

The programmed controller 80 may further be configured withsynchronization logic and programmed according to yet another embodimentto generate a control signal that activates an auxiliary/spare generatorto provide a parallel operation capability.

While the invention has been described in terms of various specificembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theclaims.

1. A motor-driven pump system, comprising: a single off-grid primemover, the single off-grid prime mover comprising a rotationaldriveshaft and operating in response to a throttle or fuel inputcontroller to control a rotation speed of the prime mover driveshaft;one or more electric power generators driven by the single off-gridprime mover rotational driveshaft to generate AC or DC power on a sharedelectrical bus; a plurality of variable speed motors configured toreceive electric power via the shared electrical bus; and a plurality ofpumps connected to a shared high pressure manifold and driven by theplurality of variable speed motors to generate a desired wellheadpressure via the shared high pressure manifold.
 2. The motor-driven pumpsystem according to claim 1, wherein the throttle or fuel inputcontroller is integrated with the motor-driven pump system.
 3. Themotor-driven pump system according to claim 1, wherein the throttle orfuel input controller is remote from the motor-driven pump system. 4.The motor-driven pump system according to claim 1, wherein the throttleor fuel input controller is a manually operated throttle or fuel inputcontroller.
 5. The motor-driven pump system according to claim 1,wherein the throttle or fuel input controller is an automated throttleor fuel input controller.
 6. The motor-driven pump system according toclaim 1, wherein the single off-grid prime mover comprises at least oneof a reciprocating engine, a turbine, or a rotational energy source. 7.The motor-driven pump system according to claim 1, wherein the one ormore electric power generators comprise at least one of a permanentmagnet generator, a wound-field synchronous generator, a DC generator,an induction generator, a synchronous reluctance generator, a homopolarinduction alternator, or an AC generator.
 8. The motor-driven pumpsystem according to claim 7, further comprising a generator exciter forproviding excitation to the generator.
 9. The motor-driven pump systemaccording to claim 8, further comprising a flow sensor configured tomonitor a flow rate associated with the plurality of pumps and togenerate a sensor signal in response thereto.
 10. The motor-driven pumpsystem according to claim 9, wherein the throttle or fuel inputcontroller is programmed to control the excitation of the generatorexciter in response to the flow sensor signal.
 11. The motor-driven pumpsystem according to claim 1, further comprising at least one pressuresensor configured to monitor the wellhead pressure.
 12. A method ofoperating a motor-driven pump system, the method comprising: controllinga driveshaft rotation speed of an off-grid prime mover in response to athrottle or fuel input controller, wherein operation of the throttle orfuel input controller is based on known characteristics of a pumpingload; controlling AC or DC power supplied to a shared electrical busfrom at least one electric power generator in response to a driveshaftrotation speed of the off-grid prime mover; controlling a speed of aplurality of variable speed motors directly in response to the AC or DCpower on the electrical bus shared by the plurality of variable speedmotors; and generating a desired wellhead pressure or pumping load flowrate via a plurality of pumps driven by the plurality of variable speedmotors and connected to a high pressure manifold shared by the pluralityof pumps.
 13. The method of operating a motor-driven pump systemaccording to claim 12, wherein controlling a driveshaft rotation speedof an off-grid prime mover comprises controlling a driveshaft rotationspeed of at least one of a reciprocating engine, a turbine engine, or arotational energy source.
 14. The method of operating a motor-drivenpump system according to claim 12, wherein controlling the AC or DCpower supplied to the shared electrical bus comprises controlling an ACpower output of at least one of a wound-field synchronous generator, apermanent magnet generator, a DC generator, an induction generator, asynchronous reluctance generator, a homopolar induction alternator, oran AC generator.
 15. The method of operating a motor-driven pump systemaccording to claim 14, wherein controlling an AC power output of thewound-field synchronous generator comprises controlling an AC poweroutput of a wound-field exciter.
 16. The method of operating amotor-driven pump system according to claim 15, further comprisingcontrolling the AC power output of the wound-field exciter in responseto the driveshaft rotation speed of the off-grid prime mover.
 17. Themethod of operating a motor-driven pump system according to claim 14,further comprising controlling the AC power output of the permanentmagnet generator in response to the driveshaft rotation speed of theoff-grid prime mover.
 18. The method of operating a motor-driven pumpsystem according to claim 12, further comprising linking the generatedAC or DC power to the variable speed motor via a transformer.
 19. Themethod of operating a motor-driven pump system according to claim 12,further comprising operating the throttle or fuel input controller inresponse to known characteristics of a pumping load, wherein the knowncharacteristics of the pumping load are a quadratic function of motorspeed.